JAL use the 747-400D between Tokyo and Osaka. 600 seats, no galleys, 40 minute sectors.
Really?
I think the largest seat count I´ve heard of was 594, in Japan, and I understand it no longer flies. Currently the biggest plane in the world is the 747-400 of Corsair - with 587 seats. And they have long sectors (what about galleys?).
Oh, and technically propeller planes should be easier to stretch than jets. Low cruise Mach should allow thick, unswept wings, which would offer better resistance to bending and twisting at less structural mass. How would a Brabazon wing compare to an A380 wing?